A container is simply a widget
   that is able to contain another widget.  The vast majority of widgets fall
   into this class; obvious examples include GtkWindow, 
   GtkTable and GtkBox.  Other
   than this property they are exactly the same as any other widget, and may
   be added to other containers as well as supporting child widgets of their
   own.  They are derived from the GtkContainer base
   class, which is itself derived from the GtkWidget 
   class.